In this invention, the polyolefin is coated with a blend of an additive and a polyolefin carrier or vehicle. Relatively low molecular weight of the carrier resin is preferable.
The method traditionally employed for incorporating dry additives into polymers involves blending the polymer with the additive, using an extruder, Banbury mixer or other mixing apparatus. Disadvantages of the current techniques include appreciable downtime for cleaning of the blending equipment, high electricity costs of running the extruders, and maintenance costs.
A variety of patents teach methods for blending additives with polymers. For example, U.S. Pat. No. 3,591,409, teaches a method for preparing a composition by high intensity mechanical blending of a mixture of particulate thermoplastic resin, hydrocarbon wax, and particulate solid material at high temperatures. This teaching requires that all ingredients be placed in the mixer at once, without separate pretreatment of any one of the components of the mixture.
In contrast with this conventional teaching, it has been discovered that a formulation can be prepared which consists of additives and a molten polyolefin carrier; the mixture can be sprayed onto the polymer particles (to be stabilized) through a fluidized bed coating technique, wherein the polymer is partially coated and discrete particles are formed.
A U.S. Pat. No. 3,271,185 to Phillips Petroleum Co. discloses a process for stabilizing polymers wherein a vaporizable antioxidant is added in a fluidized bed to the polymer.
Japanese Patent No. SHO[59]-150719 is directed towards a process where polypropylene is mixed at a high velocity with a stabilizer whose melting temperature must be below 162.degree. C.
A U.S. Pat. No. 4,452,928 to El Paso Polyolefins Co. discloses the use of solvents which must be removed after adding the stabilizer to the polymer. The teaching suggests that the polymer be purified and the solvent recycled to obtain good benefits from the disclosed process.
A European Patent, No. 200,684 to Ciba-Geigy discloses the stabilization of spherical polyolefins by treatment with emulsion or dispersion containing stabilizers and certain amines. Emulsifiers, such as sorbitan monooleate, are used in the disclosed process. The sorbitan monooleate remains in the resultant polypropylene.
It has long been desired to find a method of cheaply blending additives into polymers without (1) having to additionally compound the polymer with the additive using energy-intensive mixers like extruders, and (2) resorting to a process requiring the use of solvents.
The present invention, directed toward a fluidized bed coating process, has lower energy and capital cost requirements compared to the single or twin-screw extrusion technology, and attempts to provide a new solution to an old problem.